Pressure sensor device and assembly method

ABSTRACT

A semiconductor sensor device is assembled using a pre-molded lead frame having first and second die flags. The first die flag includes a cavity. A pressure sensor die (P-cell) is mounted within the cavity and a master control unit die (MCU) is mounted to the second flag. The P-cell and MCU are electrically connected to leads of the lead frame with bond wires. The die attach and wire bonding steps are each done in a single pass. A mold pin is placed over the P-cell and then the MCU is encapsulated with a mold compound. The mold pin is removed leaving a recess that is next filled with a gel material. Finally a lid is placed over the P-cell and gel material. The lid includes a hole that that exposes the gel-covered active region of the pressure sensor die to ambient atmospheric pressure outside the sensor device.

BACKGROUND OF THE INVENTION

The present invention relates generally to semiconductor sensor devicesand, more particularly to a method of assembling a semiconductorpressure sensor device.

Semiconductor sensor devices, such as pressure sensors, are well known.Such devices use semiconductor pressure sensor dies. These dies aresusceptible to mechanical damage during packaging and environmentaldamage when in use, and thus they must be carefully packaged. Further,pressure sensor dies, such as piezo resistive transducers (PRTs) andparameterized layout cells (P-cells), do not allow full encapsulationbecause that would impede their functionality.

FIG. 1(A) shows a cross-sectional side view of a conventional packagedsemiconductor sensor device 100 having a metal lid 104. FIG. 1(B) showsa perspective top view of the sensor device 100 partially assembled, andFIG. 1(C) shows a perspective top view of the lid 104.

As shown in FIG. 1, a pressure sensor die (P-cell) 106,acceleration-sensing die (G-cell) 108, and master control unit die (MCU)110 are mounted to a lead frame flag 112, electrically connected to leadframe leads 118 with bond wires (not shown), and covered with apressure-sensitive gel 114, which enables the pressure of the ambientatmosphere to reach the pressure-sensitive active region on the top sideof P-cell 106, while protecting all of the dies 106, 108, 110 and thebond wires from mechanical damage during packaging and environmentaldamage (e.g., contamination and/or corrosion) when in use. The entiredie/substrate assembly is encased in mold compound 102 and covered bythe lid 104, which has a vent hole 116 that exposes the gel-coveredP-cell 106 to ambient atmospheric pressure outside the sensor device100.

One problem with the sensor device 100 is the high manufacturing costdue to the use of a pre-molded lead frame, the metal lid 104, and thelarge volume of pressure-sensitive gel 114. Accordingly, it would beadvantageous to have a more economical way to package dies insemiconductor sensor devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure are illustrated by way of exampleand are not limited by the accompanying figures, in which likereferences indicate similar elements. Elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the thicknesses of layers and regions maybe exaggerated for clarity.

FIG. 1 shows a conventional packaged semiconductor sensor device havinga metal lid;

FIG. 2 shows a semiconductor sensor device in accordance with anembodiment of the present invention; and

FIGS. 3-7 illustrate a method of assembling the sensor device of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Detailed illustrative embodiments of the present disclosure aredisclosed herein. However, specific structural and functional detailsdisclosed herein are merely representative for purposes of describingexample embodiments of the present disclosure. Embodiments of thepresent disclosure may be embodied in many alternative forms and shouldnot be construed as limited to only the embodiments set forth herein.Further, the terminology used herein is for the purpose of describingparticular embodiments only and is not intended to be limiting ofexample embodiments of the disclosure.

As used herein, the singular forms “a,” “an,” and “the,” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It further will be understood that the terms “comprises,”“comprising,” “has,” “having,” “includes,” and/or “including” specifythe presence of stated features, steps, or components, but do notpreclude the presence or addition of one or more other features, steps,or components. It also should be noted that, in some alternativeimplementations, the functions/acts noted may occur out of the ordernoted in the figures. For example, two figures shown in succession mayin fact be executed substantially concurrently or may sometimes beexecuted in the reverse order, depending upon the functionality/actsinvolved.

One embodiment of the disclosure is a method for manufacturing asemiconductor sensor device, and another embodiment is the resultingsemiconductor sensor device. At least two dies, comprising (i) apressure sensor die having a pressure-sensitive active region and (ii)at least one other die, are die-bonded to a lead frame. The at least twodies are wire-bonded to corresponding leads of the lead frame using bondwires. A mold pin is placed over the pressure sensor die and its bondwires. Mold compound is applied to encapsulate the at least one otherdie and its bond wires. The mold pin is removed leaving a recess in themold compound surrounding the pressure sensor die and its bond wires.Pressure-sensitive gel is applied in the recess to cover the activeregion of the pressure sensor die and its bond wires.

Another embodiment of the disclosure is a semiconductor sensor devicecomprising (i) a pre-molded lead frame, (ii) two or more dies includinga pressure sensor die and at least one other die mounted to the leadframe, (iii) bond wires electrically interconnecting the two or moredies and the lead frame, (iv) mold compound encapsulating the at leastone other die and its associated bond wires, and (v) pressure-sensitivegel covering an active region of the pressure sensor die and itsassociated bond wires. At least one lead of the lead frame is wirebonded to both (i) the pressure sensor die and (ii) the at least oneother die, and the pressure sensor die is located in a cavity of a flagof the lead frame.

FIGS. 2(A) and 2(B) respectively show a cross-sectional side view and atop plan view of a packaged semiconductor sensor device 200 inaccordance with an embodiment of the disclosure. The exemplaryconfiguration of sensor device 200 forms a no-leads type package such asa quad flat no-leads (QFN) package. Note that alternative embodimentsare not limited to QFN packages, but can be implemented for otherpackage types, such as (without limitation) ball grid array (BGA)packages, molded array packages (MAP), and quad flat pack (QFP) or otherleaded packages.

Sensor device 200 includes a pressure sensor die 202 and an ASIC die 204mounted to (e.g., physically attached and electrically coupled to) apre-molded lead frame 206, and an acceleration-sensing die 208 mountedto ASIC die 204. Pressure sensor die (aka P-cell) 202 is designed tosense ambient atmospheric pressure, while acceleration-sensing die (akaG-cell) 208 is designed to sense gravity or acceleration in one, two, orall three axes, depending on the particular implementation. ASIC die 204functions as the master control unit (MCU) for P-cell 202 and G-cell 208by, for example, controlling the operations of and processing signalsgenerated by those two sensor dies. ASIC die 204 is synonymouslyreferred to herein as MCU 204. Note that, in some embodiments, ASIC die204 may implement both the functionality of an MCU and that of one ormore other sensors, such as an acceleration-sensing G-cell, in whichlatter case, G-cell 208 may be omitted.

Pre-molded lead frame 206 comprises electrically conductive leads 210embedded in an electrically insulating mold compound 212. Lead 210 maybe formed of copper, an alloy of copper, a copper plated iron/nickelalloy, plated aluminum, or the like. Often, copper leads are pre-platedfirst with a nickel base layer, then a palladium mid layer, and finallywith a very thin, gold upper layer. Mold compound 212 may be an epoxy orother suitable material.

Conventional, electrically insulating die-attach adhesive 224 may beused to attach (i) P-cell 202 and MCU 204 to lead frame 206 and (ii)G-cell 208 to MCU 204. Those skilled in the art will understand thatsuitable alternative means, such as die-attach tape, may be used toattach some or all of these dies. P-cell 202, MCU 204, and G-cell 208are well known components of semiconductor sensor devices and thusdetailed descriptions thereof are not necessary for a completeunderstanding of the disclosure.

The electrical interconnection between P-cell 202 and MCU 204 isprovided via one or more shared lead(s) 210A of lead frame 206 byrespective, associated bond wires 214 wire-bonded between (i) bond padson P-cell 202 and MCU 204 and (ii) lead(s) 210A using a suitable, knownwire-bonding process and suitable, known wire-bonding equipment.Similarly, the electrical interconnection between MCU 204 and G-cell 208is provided by wire-bonding between other bond pads on MCU 204 and bondpads on G-cell 208. Furthermore, the electrical interconnection betweenMCU 204 and the outside world is provided via one or more lead(s) 210Bof lead frame 206 by bond wires 214 wire-bonded between still other padson MCU 204 and lead(s) 210B. Bond wires 214 are formed from a conductivematerial such as aluminium, gold, or copper, and may be either coated oruncoated. Note that, in alternative designs, G-cell 208 can beelectrically connected to MCU 204 using suitable flip-chip, solder-bumptechniques instead of or in addition to wire-bonding.

MCU 204, G-cell 208, and their associated bond wires 214 areencapsulated within a suitable mold compound 216. The mold compound maybe a plastic, an epoxy, a silica-filled resin, a ceramic, a halide-freematerial, the like, or combinations thereof, as is known in the art.

A pressure-sensitive gel material 218, such as a silicon-based gel, isdeposited over P-cell 214 and its associated bond wires 214, fillingmost of the recess formed in mold compound 216 around P-cell 214. Notethat, in alternative implementations, less of gel material 218 may beapplied within the recess as long as the pressure-sensitive activeregion (typically on the top side) of P-cell 214 and its associated bondwires are covered by the gel. Pressure-sensitive gel material 218enables the pressure of the ambient atmosphere to reach the activeregion of P-cell 202, while protecting P-cell 202 and its associatedbond wires 214 from (i) mechanical damage during packaging and (ii)environmental damage (e.g., contamination and/or corrosion) when in use.Examples of suitable pressure-sensitive gel material 218 are availablefrom Dow Corning Corporation of Midland, Mich. The gel material may bedispensed with a nozzle of a conventional dispensing machine, as isknown in the art.

A lid 220 having an opening or vent hole 222 is mounted over thegel-covered P-cell 202 fitting snugly into a seat formed within moldcompound 216, thereby providing a protective cover for the P-cell. Venthole 222 allows the ambient atmospheric pressure immediately outsidesensor device 200 to reach (i) the pressure-sensitive gel material 218and therethrough (ii) the active region of P-cell 202. Although showncentered in FIG. 2, vent hole 222 can be located anywhere within thearea of lid 220. Vent hole 222 may be (pre-)formed in the lid by ansuitable fabrication process such as drilling or punching.

Lid 220 is formed of a durable and stiff material, such as stainlesssteel, plated metal, or polymer, so that P-cell 202 is protected. Lid220 is sized and shaped depending on the size and shape of P-cell 202mounted to the lead frame under the lid. Accordingly, depending on theimplementation, the lid may have any suitable shape, such as round,square, or rectangular.

Sensor device 200 can be manufactured with less cost than comparablesensor devices, like those based on the conventional design of sensordevice 100 of FIG. 1, because sensor device 200 can be manufactured witha smaller lid and with less pressure-sensitive gel.

FIGS. 3-7 illustrate one possible process for manufacturing sensordevice 200 of FIG. 2.

In particular, FIGS. 3(A), 3(B), and 3(C) respectively show across-sectional side view, a top plan view, and a three-dimensional (3D)perspective view of pre-molded lead frame 206 having electricallyconductive leads 210 embedded in electrically insulating mold compound212. Lead frame 206 also has a shallow recess 302 for receiving P-cell202 of FIG. 2. The purpose of recess 302 is to prevent the die-attachmaterial (e.g., 224 in FIG. 2) from flowing out to the wire-bonding areaof leads 210.

FIGS. 4(A) and 4(B) respectively show a cross-sectional side view and atop plan view of (i) P-cell 202 and MCU 204 mounted on and wire-bondedto lead frame 206 of FIG. 3 and (ii) G-cell 208 mounted on andwire-bonded to MCU 204. Note that the attachment or die-bonding of allof P-cell 202, MCU 204, and G-cell 204 can be achieved in a singledie-bonding process step that includes the curing of the epoxy or othersubstance (e.g., die-attach tape) used to mount all of those dies in asingle pass through a curing cycle (e.g., comprising heating and/or UVirradiation). Furthermore, (i) P-cell 202 and MCU 204 can beelectrically connected to lead frame 206 and (ii) G-cell 208 can beelectrically connected to MCU 204 all in a single pass though awire-bonding cycle (or in a single wire-bonding process step).

FIGS. 5(A) and 5(B) respectively show a cross-sectional side view and apartial X-ray, top plan view of the sub-assembly of FIG. 4 with mold pin502 placed over P-cell 202. Mold pin 502 comprises (i) a lower portion504 defining a cavity 506 that accommodates P-cell 202 as well as itsassociated bond wires 214 and (ii) an upper portion 508 whose outerdimensions are slightly larger than the outer dimensions of lowerportion 504. In FIG. 5(B), the outline labeled 504 represents theperiphery of the lower portion of mold pin 502 resting on lead frame206. Note that the existence of the larger, upper portion 508 isoptional.

FIGS. 6(A) and 6(B) respectively show a cross-sectional side view and apartial X-ray, top plan view of the sub-assembly of FIG. 5 after theaddition of mold compound 216 to encapsulate everything in thesub-assembly of FIG. 5 that is outside of the cavity defined by mold pin502. One way of applying the mold compound 216 is using a mold insert ofa conventional injection-molding machine, as is known in the art. Themolding material is typically applied as a liquid polymer, which is thenheated to form a solid by curing in a UV or ambient atmosphere. Themolding material can also be a solid that is heated to form a liquid forapplication and then cooled to form a solid mold. Subsequently, an ovenis used to cure the molding material to complete the cross linking ofthe polymer. In alternative embodiments, other encapsulating processesmay be used. The mold pin 502 prevents mold compound 216 from seepinginside the cavity 506 and reaching the P-cell 202.

In the implementation shown in FIG. 6, the mold compound 216 is appliedto a height that is slightly higher than the lower portion 504 of themold pin 502, such that the mold compound 216 extends past the bottom ofthe upper portion 508 of the mold pin 502. After encapsulation, the moldpin 502 is removed from the sub-assembly of FIG. 6, leaving behind arecess or cavity within the mold compound 216 surrounding the P-cell 506and its associated bond wires 214.

FIGS. 7(A) and 7(B) respectively show a cross-sectional side view and apartial X-ray, top plan view of the sub-assembly of FIG. 6 after theremoval of the mold pin 502 and after the subsequent addition ofpressure-sensitive gel material 218, which covers the P-cell 202 and itsassociated bond wires 214. In the implementation shown in FIG. 7, thegel material 218 is applied up to the top of the bottom,smaller-dimensioned portion of the recess formed by the lower portion504 of the mold pin 502, leaving the top, larger-dimensioned portion ofthe recess formed by the upper portion 508 of the mold pin 502 unfilled.

Referring again to FIG. 2, after the application of gel material 218,the lid 220 is mounted over the P-cell 202 and gel material 218 to formthe final assembly of the sensor device 200. Note that the upper portion508 of the mold pin 502 has substantially the same outer dimensions asthe lid 220 so that the lid 220 fits snugly within the seat formed inmold compound 216 by the upper portion 508. The lid 220 preferably liesflush with a top (outer) surface of the mold compound 216. Note that,for implementations in which the mold pin 502 does not have alarger-dimensioned, upper portion, but rather only a single-dimensionedportion, the lid 220 is fabricated to allow it to be press-fit into therecess formed over the P-cell 202.

The shapes of the leads 210 of the lead frame 206, specifically lead(s)210A, enable the indirect electrical interconnection of the P-cell 202and MCU 204 by wire bonding both dies 202, 204 to one or more sharedlead(s) 210A. This lead sharing, in turn, allows the mold pin 502 to beplaced over the P-cell 202 in a way that does not impinge on either thebond wires 214 connecting the P-cell 202 to shared lead(s) 210A or thebond wires 214 connecting the MCU 204 to the same shared lead(s) 210A.In this way, the bond wires associated with the MCU 204 can beencapsulated by the mold compound 216, while the bond wires associatedwith the P-cell 202 are covered with the gel material 218. Thesefeatures enable the sensor device 200 to be manufactured with only asingle die-bonding cycle and only a single wire-bonding cycle.

Although not depicted in the drawings, in practice, a plurality ofsensor devices are formed simultaneously by using a lead frame sheetthat has a two-dimensional array of the lead frames, and then the diebonding and wire bonding steps are performed on all of the lead framesin the array. Similarly, all of the separate devices are encapsulatedwith the molding compound at the same time too. After assembly, e.g.,using the process depicted in FIGS. 3-7, the multiple sensor devices areseparated, e.g., in a singulation process involving a saw or laser, toform individual instances of the sensor device 200.

As used herein, the term “mounted to” as in “a first die mounted to alead frame” covers situations in which the first die is mounted directlyto the lead frame with no other intervening dies (as in the mounting ofP-cell 202 to lead frame 206 in FIG. 2) as well as situations in whichthe first die is directly mounted to another die, which is itselfmounted directly to the lead frame (as in the mounting of G-cell 208 tolead frame 206 via MCU 204 in FIG. 2). Note that “mounted to” alsocovers situations in which there are two or more intervening diesbetween the first die and lead frame.

Although FIG. 2 shows sensor devices 200 having a P-cell and a G-cell,those skilled in the art will understand that, in alternativeembodiments, the G-cell and its corresponding bond wires may be omitted.

Although FIG. 2 shows an embodiment in which a G-cell is mounted to theMCU with the electrical interconnection provided by wire-bonding, thoseskilled in the art will understand that the electrical interconnectionbetween such dies can, alternatively or additionally, be provided byappropriate flip-chip assembly techniques. According to thesetechniques, two semiconductor dies are electrically interconnectedthrough flip-chip bumps attached to one of the semiconductor dies. Theflip-chip bumps may include solder bumps, gold balls, molded studs, orcombinations thereof. The bumps may be formed or placed on asemiconductor die using known techniques such as evaporation,electroplating, printing, jetting, stud bumping, and direct placement.The semiconductor die is flipped, and the bumps are aligned withcorresponding contact pads of the other die.

By now it should be appreciated that there has been provided an improvedpackaged semiconductor sensor device and a method of forming theimproved packaged semiconductor sensor device. Circuit details are notdisclosed because knowledge thereof is not required for a completeunderstanding of the invention.

Although the invention has been described using relative terms such as“front,” “back,” “top,” “bottom,” “over,” “above,” “under” and the likein the description and in the claims, such terms are used fordescriptive purposes and not necessarily for describing permanentrelative positions. It is understood that the terms so used areinterchangeable under appropriate circumstances such that theembodiments of the disclosure described herein are, for example, capableof operation in other orientations than those illustrated or otherwisedescribed herein.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements. Further, the use of introductoryphrases such as “at least one” and “one or more” in the claims shouldnot be construed to imply that the introduction of another claim elementby the indefinite articles “a” or “an” limits any particular claimcontaining such introduced claim element to inventions containing onlyone such element, even when the same claim includes the introductoryphrases “one or more” or “at least one” and indefinite articles such as“a” or “an.” The same holds true for the use of definite articles.

Although the disclosure is described herein with reference to specificembodiments, various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope of thepresent invention. Any benefits, advantages, or solutions to problemsthat are described herein with regard to specific embodiments are notintended to be construed as a critical, required, or essential featureor element of any or all the claims.

It should be understood that the steps of the exemplary methods setforth herein are not necessarily required to be performed in the orderdescribed, and the order of the steps of such methods should beunderstood to be merely exemplary. Likewise, additional steps may beincluded in such methods, and certain steps may be omitted or combined,in methods consistent with various embodiments of the invention.

Although the elements in the following method claims, if any, arerecited in a particular sequence with corresponding labeling, unless theclaim recitations otherwise imply a particular sequence for implementingsome or all of those elements, those elements are not necessarilyintended to be limited to being implemented in that particular sequence.

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment can be included in at least one embodiment of theinvention. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment, nor are separate or alternative embodiments necessarilymutually exclusive of other embodiments. The same applies to the term“implementation.”

The embodiments covered by the claims in this application are limited toembodiments that (1) are enabled by this specification and (2)correspond to statutory subject matter. Non enabled embodiments andembodiments that correspond to non statutory subject matter areexplicitly disclaimed even if they fall within the scope of the claims.

1. A semiconductor sensor device comprising: a pre-molded lead framehaving a plurality of leads and at least first and second die flags,wherein the first die flag has a cavity formed therein; a pressuresensor die attached within the cavity of the first die flag, and atleast one other die mounted to second die flag; first bond wireselectrically interconnecting the pressure sensor die with first ones ofthe leads of the lead frame, and second bond wires electricallyinterconnecting the at least one other die with second ones of the leadsof the lead frame; mold compound encapsulating the at least one otherdie and the second bond wires; pressure sensitive gel covering an activeregion of the pressure sensor die and the first bond wires; and a lidmounted over the pressure sensor die, wherein the lid has a hole thatexposes the gel covered active region of the pressure sensor die toambient atmospheric pressure outside the sensor device.
 2. The sensordevice of claim 1, wherein at least one lead of the lead frame is wirebonded to both the pressure sensor die and the at least one other die.3. The sensor device of claim 1, wherein the lid fits into a seat formedin the mold compound.
 4. The sensor device of claim 1, wherein the atleast one other die comprises a master control unit (MCU).
 5. The sensordevice of claim 4, further comprising an acceleration sensor die mountedto the ASIC die, wherein the acceleration sensor die is electricallyconnected to the MCU with third bond wires.
 6. A method for assembling asemiconductor sensor device, the method comprising: providing apre-molded lead frame having a first die flag and a second die flag,wherein the first die flag has cavity formed therein; die bonding apressure sensor die within the cavity of the first die flag and a mastercontrol unit die (MCU) to a surface of the second die flag, wherein diebonding of the pressure sensor die and the MCU is done in a single pass;electrically connecting the pressure sensor die to first leads of thelead frame with first bond wires, and electrically connecting the MCU tosecond leads of the lead frame with second bond wires; placing a moldpin over the pressure sensor die and the first bond wires; encapsulatingthe MCU and the second bond wires with a mold compound; removing themold pin whereby a recess in the mold compound surrounding the pressuresensor die is formed; and applying pressure sensitive gel in the recessto cover an active region of the pressure sensor die.
 7. The method ofclaim 6, further comprising: mounting a lid over the gel-coveredpressure sensor die, wherein the lid has a hole that exposes thegel-covered active region of the pressure sensor die to ambientatmospheric pressure outside the sensor device.
 8. The method of claim7, wherein the lid is mounted within recesses in the mold compound suchthat a top surface of the lid is flush with a top surface of the moldcompound.
 9. The method of claim 6, further comprising mounting anaccelerometer die on a surface of the MCU and electrically connectingthe accelerometer die with the MCU with third bond wires.
 10. The methodof claim 6, wherein the mold pin comprises a lower portion having anouter dimension and defining a cavity and an upper portion having anouter dimension larger than the outer dimension of the lower portion;and the encapsulating step includes applying the mold compound to alevel higher than the lower portion of the mold pin, such that the upperportion forms a seat in the mold compound configured to snugly receivethe lid.
 11. The method of claim 6, wherein at least one of the firstleads is a same lead as one of the second leads.
 12. The method of claim6, further comprising: separating the sensor device from one or moreother instances of the sensor device assembled simultaneously with thesensor device.